Rockwell Automation Low-Voltage Switchgear and Controlgear User Manual
Page 115
IEC 60947-4-1 provides various trip classes (
) for motor protection relays in order to
adapt the protective devices to the starting conditions. The limiting values with tighter tolerances
“E” have been introduced for electronic protective relays. Under heavy-duty starting conditions,
electronic motor protective devices can be advantageously used since they can be adjusted to
the specific starting conditions (see Section
). Solutions with thermal motor protection
relays and saturation current transformers, bypassing of the protective relays during starting or
use of a separate protective relay for the starting are thus obsolete.
It should be noted that with heavy-duty starting, it may be necessary to increase the cross-
section of wiring of the starter components and of the motor. Thus IEC 60947-4-1 stipulates in
the test conditions for protective relays of Classes 10, 20, 30 and 40 and for protective relays,
for which a maximum tripping time > 40 s is specified, that the tests - among other things - must
be performed with conductor cross sections suitable for 125 % of the current setting on the
relays.
Trip class
Tripping time at 7.2 · I
e
(normal tolerance)
s
Tripping time at 7.2 · I
e
(tighter tolerances „E")
s
2 -
T
p
≤ 2
3 -
2
<
T
p
≤ 3
5 -
3
<
T
p
≤ 5
10A 2
<
T
p
≤ 10
-
10 4
<
T
p
≤ 10
5 < T
p
≤ 10
20 6
<
T
p
≤ 20
10 < T
p
≤ 20
30 9
<
T
p
≤ 30
20 < T
p
≤ 30
40 -
30
<
T
p
≤ 40
Tab. 4.1-3
Trip classes of overload relays in accordance with IEC 60947-4-1 am 2. The trip class number stands for
the longest permissible tripping time at 7.2 · I
e
from a cold state.
Copyright © IEC, Geneva, Switzerland, www.iec.ch
Motor protection in hazardous areas
Motor protective devices for the protection of motors of the protection type “Increased Safety”
EEx e must comply with the standards and regulations like discussed in Section
. The
motor protective devices themselves are not explosion-protected and may therefore not be
installed in the hazardous zones.
4.1.2.3
Overload and overtemperature protection by measurement of current
and measurement of temperature
The obvious way to identify excess temperatures is by directly measuring them. Thus all factors
are included that influence the temperature at the measuring location – e.g. the ambient
temperature that frequently varies within a wide range and that is often not taken into account
by current-measuring protective devices or obstructed cooling. When measuring the current, a
simulation of the temperature rise is carried out and a worst-case scenario is created with
respect to the ambient temperature conditions. It is thereby assumed that the ambient tempera-
ture of the protected object corresponds to the maximum permissible temperature. This
reference temperature is defined for motors in accordance with IEC 60034 to 40 °C at site
altitudes of up to 1000 m.
Temperature sensors (for example PTC) are frequently being used for measuring the tempera-
ture of motor windings and have proven its worth in practice. The effect of measurement delays
at very rapid temperature rises (for example with a locked rotor condition) is only adverse for
thermally especially critical motors (for example submersible pump motors) or large machines in
which the rotor is the thermally critical component (
). However temperature measure-
ment is not always appropriate, possible or at least it can be very expensive. In rotor-critical
LVSAM-WP001A-EN-P - April 2009
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